5.24 Declaring Attributes of Functions

In GNU C, you declare certain things about functions called in your program
which help the compiler optimize function calls and check your code more
carefully.

The keyword __attribute__ allows you to specify special
attributes when making a declaration. This keyword is followed by an
attribute specification inside double parentheses. The following
attributes are currently defined for functions on all targets:
noreturn, returns_twice, noinline, always_inline,
flatten, pure, const, nothrow, sentinel,
format, format_arg, no_instrument_function,
section, constructor, destructor, used,
unused, deprecated, weak, malloc,
alias, warn_unused_result, nonnull
and externally_visible. Several other
attributes are defined for functions on particular target systems. Other
attributes, including section are supported for variables declarations
(see Variable Attributes) and for types (see Type Attributes).

You may also specify attributes with `__' preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __noreturn__ instead of noreturn.

declares `f' to be a weak alias for `__f'. In C++, the
mangled name for the target must be used. It is an error if `__f'
is not defined in the same translation unit.

Not all target machines support this attribute.

always_inline

Generally, functions are not inlined unless optimization is specified.
For functions declared inline, this attribute inlines the function even
if no optimization level was specified.

flatten

Generally, inlining into a function is limited. For a function marked with
this attribute, every call inside this function will be inlined, if possible.
Whether the function itself is considered for inlining depends on its size and
the current inlining parameters. The flatten attribute only works
reliably in unit-at-a-time mode.

cdecl

On the Intel 386, the cdecl attribute causes the compiler to
assume that the calling function will pop off the stack space used to
pass arguments. This is
useful to override the effects of the -mrtd switch.

const

Many functions do not examine any values except their arguments, and
have no effects except the return value. Basically this is just slightly
more strict class than the pure attribute below, since function is not
allowed to read global memory.

Note that a function that has pointer arguments and examines the data
pointed to must not be declared const. Likewise, a
function that calls a non-const function usually must not be
const. It does not make sense for a const function to
return void.

The attribute const is not implemented in GCC versions earlier
than 2.5. An alternative way to declare that a function has no side
effects, which works in the current version and in some older versions,
is as follows:

typedef int intfn ();
extern const intfn square;

This approach does not work in GNU C++ from 2.6.0 on, since the language
specifies that the `const' must be attached to the return value.

constructor

destructor

The constructor attribute causes the function to be called
automatically before execution enters main (). Similarly, the
destructor attribute causes the function to be called
automatically after main () has completed or exit () has
been called. Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.

These attributes are not currently implemented for Objective-C.

deprecated

The deprecated attribute results in a warning if the function
is used anywhere in the source file. This is useful when identifying
functions that are expected to be removed in a future version of a
program. The warning also includes the location of the declaration
of the deprecated function, to enable users to easily find further
information about why the function is deprecated, or what they should
do instead. Note that the warnings only occurs for uses:

On Microsoft Windows targets and Symbian OS targets the
dllexport attribute causes the compiler to provide a global
pointer to a pointer in a DLL, so that it can be referenced with the
dllimport attribute. On Microsoft Windows targets, the pointer
name is formed by combining _imp__ and the function or variable
name.

You can use __declspec(dllexport) as a synonym for
__attribute__ ((dllexport)) for compatibility with other
compilers.

On systems that support the visibility attribute, this
attribute also implies “default” visibility, unless a
visibility attribute is explicitly specified. You should avoid
the use of dllexport with “hidden” or “internal”
visibility; in the future GCC may issue an error for those cases.

Currently, the dllexport attribute is ignored for inlined
functions, unless the -fkeep-inline-functions flag has been
used. The attribute is also ignored for undefined symbols.

When applied to C++ classes, the attribute marks defined non-inlined
member functions and static data members as exports. Static consts
initialized in-class are not marked unless they are also defined
out-of-class.

For Microsoft Windows targets there are alternative methods for
including the symbol in the DLL's export table such as using a
.def file with an EXPORTS section or, with GNU ld, using
the --export-all linker flag.

dllimport

On Microsoft Windows and Symbian OS targets, the dllimport
attribute causes the compiler to reference a function or variable via
a global pointer to a pointer that is set up by the DLL exporting the
symbol. The attribute implies extern storage. On Microsoft
Windows targets, the pointer name is formed by combining _imp__
and the function or variable name.

You can use __declspec(dllimport) as a synonym for
__attribute__ ((dllimport)) for compatibility with other
compilers.

Currently, the attribute is ignored for inlined functions. If the
attribute is applied to a symbol definition, an error is reported.
If a symbol previously declared dllimport is later defined, the
attribute is ignored in subsequent references, and a warning is emitted.
The attribute is also overridden by a subsequent declaration as
dllexport.

When applied to C++ classes, the attribute marks non-inlined
member functions and static data members as imports. However, the
attribute is ignored for virtual methods to allow creation of vtables
using thunks.

On the SH Symbian OS target the dllimport attribute also has
another affect—it can cause the vtable and run-time type information
for a class to be exported. This happens when the class has a
dllimport'ed constructor or a non-inline, non-pure virtual function
and, for either of those two conditions, the class also has a inline
constructor or destructor and has a key function that is defined in
the current translation unit.

For Microsoft Windows based targets the use of the dllimport
attribute on functions is not necessary, but provides a small
performance benefit by eliminating a thunk in the DLL. The use of the
dllimport attribute on imported variables was required on older
versions of the GNU linker, but can now be avoided by passing the
--enable-auto-import switch to the GNU linker. As with
functions, using the attribute for a variable eliminates a thunk in
the DLL.

One drawback to using this attribute is that a pointer to a function
or variable marked as dllimport cannot be used as a constant
address. On Microsoft Windows targets, the attribute can be disabled
for functions by setting the -mnop-fun-dllimport flag.

eightbit_data

Use this attribute on the H8/300, H8/300H, and H8S to indicate that the specified
variable should be placed into the eight bit data section.
The compiler will generate more efficient code for certain operations
on data in the eight bit data area. Note the eight bit data area is limited to
256 bytes of data.

You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.

exception_handler

Use this attribute on the Blackfin to indicate that the specified function
is an exception handler. The compiler will generate function entry and
exit sequences suitable for use in an exception handler when this
attribute is present.

far

On 68HC11 and 68HC12 the far attribute causes the compiler to
use a calling convention that takes care of switching memory banks when
entering and leaving a function. This calling convention is also the
default when using the -mlong-calls option.

On 68HC12 the compiler will use the call and rtc instructions
to call and return from a function.

On 68HC11 the compiler will generate a sequence of instructions
to invoke a board-specific routine to switch the memory bank and call the
real function. The board-specific routine simulates a call.
At the end of a function, it will jump to a board-specific routine
instead of using rts. The board-specific return routine simulates
the rtc.

fastcall

On the Intel 386, the fastcall attribute causes the compiler to
pass the first argument (if of integral type) in the register ECX and
the second argument (if of integral type) in the register EDX. Subsequent
and other typed arguments are passed on the stack. The called function will
pop the arguments off the stack. If the number of arguments is variable all
arguments are pushed on the stack.

format (archetype, string-index, first-to-check)

The format attribute specifies that a function takes printf,
scanf, strftime or strfmon style arguments which
should be type-checked against a format string. For example, the
declaration:

causes the compiler to check the arguments in calls to my_printf
for consistency with the printf style format string argument
my_format.

The parameter archetype determines how the format string is
interpreted, and should be printf, scanf, strftime
or strfmon. (You can also use __printf__,
__scanf__, __strftime__ or __strfmon__.) The
parameter string-index specifies which argument is the format
string argument (starting from 1), while first-to-check is the
number of the first argument to check against the format string. For
functions where the arguments are not available to be checked (such as
vprintf), specify the third parameter as zero. In this case the
compiler only checks the format string for consistency. For
strftime formats, the third parameter is required to be zero.
Since non-static C++ methods have an implicit this argument, the
arguments of such methods should be counted from two, not one, when
giving values for string-index and first-to-check.

In the example above, the format string (my_format) is the second
argument of the function my_print, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.

The format attribute allows you to identify your own functions
which take format strings as arguments, so that GCC can check the
calls to these functions for errors. The compiler always (unless
-ffreestanding or -fno-builtin is used) checks formats
for the standard library functions printf, fprintf,
sprintf, scanf, fscanf, sscanf, strftime,
vprintf, vfprintf and vsprintf whenever such
warnings are requested (using -Wformat), so there is no need to
modify the header file stdio.h. In C99 mode, the functions
snprintf, vsnprintf, vscanf, vfscanf and
vsscanf are also checked. Except in strictly conforming C
standard modes, the X/Open function strfmon is also checked as
are printf_unlocked and fprintf_unlocked.
See Options Controlling C Dialect.

The format_arg attribute specifies that a function takes a format
string for a printf, scanf, strftime or
strfmon style function and modifies it (for example, to translate
it into another language), so the result can be passed to a
printf, scanf, strftime or strfmon style
function (with the remaining arguments to the format function the same
as they would have been for the unmodified string). For example, the
declaration:

causes the compiler to check the arguments in calls to a printf,
scanf, strftime or strfmon type function, whose
format string argument is a call to the my_dgettext function, for
consistency with the format string argument my_format. If the
format_arg attribute had not been specified, all the compiler
could tell in such calls to format functions would be that the format
string argument is not constant; this would generate a warning when
-Wformat-nonliteral is used, but the calls could not be checked
without the attribute.

The parameter string-index specifies which argument is the format
string argument (starting from one). Since non-static C++ methods have
an implicit this argument, the arguments of such methods should
be counted from two.

The format-arg attribute allows you to identify your own
functions which modify format strings, so that GCC can check the
calls to printf, scanf, strftime or strfmon
type function whose operands are a call to one of your own function.
The compiler always treats gettext, dgettext, and
dcgettext in this manner except when strict ISO C support is
requested by -ansi or an appropriate -std option, or
-ffreestanding or -fno-builtin
is used. See Options Controlling C Dialect.

function_vector

Use this attribute on the H8/300, H8/300H, and H8S to indicate that the specified
function should be called through the function vector. Calling a
function through the function vector will reduce code size, however;
the function vector has a limited size (maximum 128 entries on the H8/300
and 64 entries on the H8/300H and H8S) and shares space with the interrupt vector.

You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.

interrupt

Use this attribute on the ARM, AVR, C4x, CRX, M32C, M32R/D, MS1, and Xstormy16
ports to indicate that the specified function is an interrupt handler.
The compiler will generate function entry and exit sequences suitable
for use in an interrupt handler when this attribute is present.

Note, interrupt handlers for the Blackfin, m68k, H8/300, H8/300H, H8S, and
SH processors can be specified via the interrupt_handler attribute.

Note, on the AVR, interrupts will be enabled inside the function.

Note, for the ARM, you can specify the kind of interrupt to be handled by
adding an optional parameter to the interrupt attribute like this:

Use this attribute on the Blackfin, m68k, H8/300, H8/300H, H8S, and SH to
indicate that the specified function is an interrupt handler. The compiler
will generate function entry and exit sequences suitable for use in an
interrupt handler when this attribute is present.

kspisusp

When used together with interrupt_handler, exception_handler
or nmi_handler, code will be generated to load the stack pointer
from the USP register in the function prologue.

long_call/short_call

This attribute specifies how a particular function is called on
ARM. Both attributes override the -mlong-calls (see ARM Options)
command line switch and #pragma long_calls settings. The
long_call attribute causes the compiler to always call the
function by first loading its address into a register and then using the
contents of that register. The short_call attribute always places
the offset to the function from the call site into the `BL'
instruction directly.

longcall/shortcall

On the Blackfin, RS/6000 and PowerPC, the longcall attribute causes
the compiler to always call this function via a pointer, just as it would if
the -mlongcall option had been specified. The shortcall
attribute causes the compiler not to do this. These attributes override
both the -mlongcall switch and, on the RS/6000 and PowerPC, the
#pragma longcall setting.

This attribute specifies how a particular function is called on MIPS.
The attribute overrides the -mlong-calls (see MIPS Options)
command line switch. This attribute causes the compiler to always call
the function by first loading its address into a register, and then using
the contents of that register.

malloc

The malloc attribute is used to tell the compiler that a function
may be treated as if any non-NULL pointer it returns cannot
alias any other pointer valid when the function returns.
This will often improve optimization.
Standard functions with this property include malloc and
calloc. realloc-like functions have this property as
long as the old pointer is never referred to (including comparing it
to the new pointer) after the function returns a non-NULL
value.

model (model-name)

On the M32R/D, use this attribute to set the addressability of an
object, and of the code generated for a function. The identifier
model-name is one of small, medium, or
large, representing each of the code models.

Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the ld24 instruction), and are
callable with the bl instruction.

Medium model objects may live anywhere in the 32-bit address space (the
compiler will generate seth/add3 instructions to load their addresses),
and are callable with the bl instruction.

Large model objects may live anywhere in the 32-bit address space (the
compiler will generate seth/add3 instructions to load their addresses),
and may not be reachable with the bl instruction (the compiler will
generate the much slower seth/add3/jl instruction sequence).

On IA-64, use this attribute to set the addressability of an object.
At present, the only supported identifier for model-name is
small, indicating addressability via “small” (22-bit)
addresses (so that their addresses can be loaded with the addl
instruction). Caveat: such addressing is by definition not position
independent and hence this attribute must not be used for objects
defined by shared libraries.

naked

Use this attribute on the ARM, AVR, C4x and IP2K ports to indicate that the
specified function does not need prologue/epilogue sequences generated by
the compiler. It is up to the programmer to provide these sequences.

near

On 68HC11 and 68HC12 the near attribute causes the compiler to
use the normal calling convention based on jsr and rts.
This attribute can be used to cancel the effect of the -mlong-calls
option.

nesting

Use this attribute together with interrupt_handler,
exception_handler or nmi_handler to indicate that the function
entry code should enable nested interrupts or exceptions.

nmi_handler

Use this attribute on the Blackfin to indicate that the specified function
is an NMI handler. The compiler will generate function entry and
exit sequences suitable for use in an NMI handler when this
attribute is present.

no_instrument_function

If -finstrument-functions is given, profiling function calls will
be generated at entry and exit of most user-compiled functions.
Functions with this attribute will not be so instrumented.

noinline

This function attribute prevents a function from being considered for
inlining.

nonnull (arg-index, ...)

The nonnull attribute specifies that some function parameters should
be non-null pointers. For instance, the declaration:

causes the compiler to check that, in calls to my_memcpy,
arguments dest and src are non-null. If the compiler
determines that a null pointer is passed in an argument slot marked
as non-null, and the -Wnonnull option is enabled, a warning
is issued. The compiler may also choose to make optimizations based
on the knowledge that certain function arguments will not be null.

If no argument index list is given to the nonnull attribute,
all pointer arguments are marked as non-null. To illustrate, the
following declaration is equivalent to the previous example:

A few standard library functions, such as abort and exit,
cannot return. GCC knows this automatically. Some programs define
their own functions that never return. You can declare them
noreturn to tell the compiler this fact. For example,

The noreturn keyword tells the compiler to assume that
fatal cannot return. It can then optimize without regard to what
would happen if fatal ever did return. This makes slightly
better code. More importantly, it helps avoid spurious warnings of
uninitialized variables.

The noreturn keyword does not affect the exceptional path when that
applies: a noreturn-marked function may still return to the caller
by throwing an exception or calling longjmp.

Do not assume that registers saved by the calling function are
restored before calling the noreturn function.

It does not make sense for a noreturn function to have a return
type other than void.

The attribute noreturn is not implemented in GCC versions
earlier than 2.5. An alternative way to declare that a function does
not return, which works in the current version and in some older
versions, is as follows:

typedef void voidfn ();
volatile voidfn fatal;

This approach does not work in GNU C++.

nothrow

The nothrow attribute is used to inform the compiler that a
function cannot throw an exception. For example, most functions in
the standard C library can be guaranteed not to throw an exception
with the notable exceptions of qsort and bsearch that
take function pointer arguments. The nothrow attribute is not
implemented in GCC versions earlier than 3.3.

pure

Many functions have no effects except the return value and their
return value depends only on the parameters and/or global variables.
Such a function can be subject
to common subexpression elimination and loop optimization just as an
arithmetic operator would be. These functions should be declared
with the attribute pure. For example,

int square (int) __attribute__ ((pure));

says that the hypothetical function square is safe to call
fewer times than the program says.

Some of common examples of pure functions are strlen or memcmp.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consecutive calls (such as feof in a multithreading environment).

The attribute pure is not implemented in GCC versions earlier
than 2.96.

regparm (number)

On the Intel 386, the regparm attribute causes the compiler to
pass arguments number one to number if they are of integral type
in registers EAX, EDX, and ECX instead of on the stack. Functions that
take a variable number of arguments will continue to be passed all of their
arguments on the stack.

Beware that on some ELF systems this attribute is unsuitable for
global functions in shared libraries with lazy binding (which is the
default). Lazy binding will send the first call via resolving code in
the loader, which might assume EAX, EDX and ECX can be clobbered, as
per the standard calling conventions. Solaris 8 is affected by this.
GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be
safe since the loaders there save all registers. (Lazy binding can be
disabled with the linker or the loader if desired, to avoid the
problem.)

sseregparm

On the Intel 386 with SSE support, the sseregparm attribute
causes the compiler to pass up to 3 floating point arguments in
SSE registers instead of on the stack. Functions that take a
variable number of arguments will continue to pass all of their
floating point arguments on the stack.

returns_twice

The returns_twice attribute tells the compiler that a function may
return more than one time. The compiler will ensure that all registers
are dead before calling such a function and will emit a warning about
the variables that may be clobbered after the second return from the
function. Examples of such functions are setjmp and vfork.
The longjmp-like counterpart of such function, if any, might need
to be marked with the noreturn attribute.

saveall

Use this attribute on the Blackfin, H8/300, H8/300H, and H8S to indicate that
all registers except the stack pointer should be saved in the prologue
regardless of whether they are used or not.

section ("section-name")

Normally, the compiler places the code it generates in the text section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections. The section
attribute specifies that a function lives in a particular section.
For example, the declaration:

extern void foobar (void) __attribute__ ((section ("bar")));

puts the function foobar in the bar section.

Some file formats do not support arbitrary sections so the section
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.

sentinel

This function attribute ensures that a parameter in a function call is
an explicit NULL. The attribute is only valid on variadic
functions. By default, the sentinel is located at position zero, the
last parameter of the function call. If an optional integer position
argument P is supplied to the attribute, the sentinel must be located at
position P counting backwards from the end of the argument list.

The attribute is automatically set with a position of 0 for the built-in
functions execl and execlp. The built-in function
execle has the attribute set with a position of 1.

A valid NULL in this context is defined as zero with any pointer
type. If your system defines the NULL macro with an integer type
then you need to add an explicit cast. GCC replaces stddef.h
with a copy that redefines NULL appropriately.

The warnings for missing or incorrect sentinels are enabled with
-Wformat.

short_call

See long_call/short_call.

shortcall

See longcall/shortcall.

signal

Use this attribute on the AVR to indicate that the specified
function is a signal handler. The compiler will generate function
entry and exit sequences suitable for use in a signal handler when this
attribute is present. Interrupts will be disabled inside the function.

sp_switch

Use this attribute on the SH to indicate an interrupt_handler
function should switch to an alternate stack. It expects a string
argument that names a global variable holding the address of the
alternate stack.

On the Intel 386, the stdcall attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments.

tiny_data

Use this attribute on the H8/300H and H8S to indicate that the specified
variable should be placed into the tiny data section.
The compiler will generate more efficient code for loads and stores
on data in the tiny data section. Note the tiny data area is limited to
slightly under 32kbytes of data.

trap_exit

Use this attribute on the SH for an interrupt_handler to return using
trapa instead of rte. This attribute expects an integer
argument specifying the trap number to be used.

unused

This attribute, attached to a function, means that the function is meant
to be possibly unused. GCC will not produce a warning for this
function.

used

This attribute, attached to a function, means that code must be emitted
for the function even if it appears that the function is not referenced.
This is useful, for example, when the function is referenced only in
inline assembly.

visibility ("visibility_type")

The visibility attribute on ELF targets causes the declaration
to be emitted with default, hidden, protected or internal visibility.

Default visibility is the normal case for ELF. This value is
available for the visibility attribute to override other options
that may change the assumed visibility of symbols.

hidden

Hidden visibility indicates that the symbol will not be placed into
the dynamic symbol table, so no other module (executable or
shared library) can reference it directly.

internal

Internal visibility is like hidden visibility, but with additional
processor specific semantics. Unless otherwise specified by the psABI,
GCC defines internal visibility to mean that the function is never
called from another module. Note that hidden symbols, while they cannot
be referenced directly by other modules, can be referenced indirectly via
function pointers. By indicating that a symbol cannot be called from
outside the module, GCC may for instance omit the load of a PIC register
since it is known that the calling function loaded the correct value.

protected

Protected visibility indicates that the symbol will be placed in the
dynamic symbol table, but that references within the defining module
will bind to the local symbol. That is, the symbol cannot be overridden
by another module.

Not all ELF targets support this attribute.

warn_unused_result

The warn_unused_result attribute causes a warning to be emitted
if a caller of the function with this attribute does not use its
return value. This is useful for functions where not checking
the result is either a security problem or always a bug, such as
realloc.

The weak attribute causes the declaration to be emitted as a weak
symbol rather than a global. This is primarily useful in defining
library functions which can be overridden in user code, though it can
also be used with non-function declarations. Weak symbols are supported
for ELF targets, and also for a.out targets when using the GNU assembler
and linker.

weakref

weakref ("target")

The weakref attribute marks a declaration as a weak reference.
Without arguments, it should be accompanied by an alias attribute
naming the target symbol. Optionally, the target may be given as
an argument to weakref itself. In either case, weakref
implicitly marks the declaration as weak. Without a
target, given as an argument to weakref or to alias,
weakref is equivalent to weak.

A weak reference is an alias that does not by itself require a
definition to be given for the target symbol. If the target symbol is
only referenced through weak references, then the becomes a weak
undefined symbol. If it is directly referenced, however, then such
strong references prevail, and a definition will be required for the
symbol, not necessarily in the same translation unit.

The effect is equivalent to moving all references to the alias to a
separate translation unit, renaming the alias to the aliased symbol,
declaring it as weak, compiling the two separate translation units and
performing a reloadable link on them.

externally_visible

This attribute, attached to a global variable or function nullify
effect of -fwhole-program command line option, so the object
remain visible outside the current compilation unit

You can specify multiple attributes in a declaration by separating them
by commas within the double parentheses or by immediately following an
attribute declaration with another attribute declaration.

Some people object to the __attribute__ feature, suggesting that
ISO C's #pragma should be used instead. At the time
__attribute__ was designed, there were two reasons for not doing
this.

It is impossible to generate #pragma commands from a macro.

There is no telling what the same #pragma might mean in another
compiler.

These two reasons applied to almost any application that might have been
proposed for #pragma. It was basically a mistake to use
#pragma for anything.

The ISO C99 standard includes _Pragma, which now allows pragmas
to be generated from macros. In addition, a #pragma GCC
namespace is now in use for GCC-specific pragmas. However, it has been
found convenient to use __attribute__ to achieve a natural
attachment of attributes to their corresponding declarations, whereas
#pragma GCC is of use for constructs that do not naturally form
part of the grammar. See Miscellaneous Preprocessing Directives.